Twisted and misaligned nanotubes long proved the bane of those looking to design CNT circuits, but Stanford researchers have surmounted these obstacles. (Source: Subhasish Mitra, Stanford University School of Engineering)

Researchers demonstrate CNT circuits built on common manufacturing processes

Carbon nanotubes (CNTs) an unusual tubular allotrope (molecular form) of pure carbon, composed of hexagonal units of SP2 bonded carbon atoms, were first observed in 1991 when studying scorched carbon soot from researchers trying to use high current discharge to form fullerenes. Since then, interest in the tiny carbon "straws" has exploded and the allotrope has become an almost wince-worthy buzzword in the U.S. university research community, suggested for use in everything from body armor to curing cancer.

I. Overcoming Crippling CNT Flaws

But you get a feeling that the Stanford University's nanofabrication lab efforts are the kind of novel research that are not merely looking to ride the buzz, but truly might be on to something great. Led by Professors Subhasish Mitra and H.-S. Philip Wong, the lab has spearheaded the challenge of producing electronic circuits out of nanotubes.

The honors came thanks to the lab's ability to demonstrate "wafer-scale" digital logic structures built from carbon nanotubes, including "arithmetic circuits and sequential storage, as well as the first monolithic three-dimensional integrated circuits with extreme levels of integration."

Carbon nanotubes have many desirable traits that could make for better computer chips someday. They are super-strong and highly efficient when acting as conductors. Yet they can be made to act as semiconductors, allowing them to be used in advanced transistor designs.

However, making such carbon nanotube circuits is a tall task. That's why the new Stanford breakthroughs are so impressive claims Carnegie Mellon University research professor Larry Pileggi, who comments, "The first CNTs wowed the research community with their exceptional electrical, thermal and mechanical properties over a decade ago, but this recent work at Stanford has provided the first glimpse of their viability to complement silicon CMOS transistors."

Using specialized production tricks the Stanford team developed ways to "ignore" two common kinds of flaws in nanotube designs -- misaligned and mis-positioned CNTs. They also developed processes to screen out yet another kind of undesirable CNTs that sabotaged past designs' efficiency -- metallic CNTs (only the semiconducting CNTs are desired for circuit elements).

II. Nearly Ready for Action in VLSI Circuits

Despite all the complexity in terms of eliminating or negating the effects of "bad" kinds of CNTs, the researchers were still able to adhere to standard semiconductor fabrication techniques, making the process applicable to very large scale integration (VLSI) -- the fundamental process used to design monolithic modern computer circuits such as central processing units (CPUs) and graphics processing units (GPUs).

Betsy Weitzman of the Focus Center Research Program at the Semiconductor Research Corporation, a North Carolina semiconductor development nonprofit, comments, "This transformative research is made all the more promising by the fact that it can co-exist with today’s mainstream silicon technologies, and leverage today’s manufacturing and system design infrastructure, providing the critical feature of economic viability."

The Stanford CNT research team poses in their "bunny suits". [Image Source: Subhasish Mitra]

Sachin S. Sapatnekar, Editor-in-Chief, IEEE Transactions on CAD praises the Stanford team's hard work and persistence in overcoming performance hurdles. He remarks, "Many researchers assumed that the way to live with imperfections in CNT manufacturing was through expensive fault-tolerance techniques. Through clever insights, Mitra and Wong have shown otherwise. Their inexpensive and practical methods can significantly improve CNT circuit robustness, and go a long way toward making CNT circuits viable."

CNTs are currently competing with alternative materials like graphene for a place in low-power super-fast circuits of the future. In future desktops and servers, such circuits will likely be complemented by other novel designs such as quantum computers (particularly useful in solving certain kinds of problems), memristor storage, or even quantum-storage.